1. Field of the Invention
The present invention relates to a color image forming apparatus for forming a color image by transferring images of different colors on a recording medium in a multi-transfer mode. More particularly, the invention relates to a color image forming apparatus which can accurately detect wrong positions of individual images transferred in a multi-transfer mode.
2. Discussion of the Related Art
Of this type of color image forming apparatuses, a color image forming apparatus capable of forming a color image at a high speed has a plurality of image forming units each including a photoreceptor drum, a laser beam scanner for writing image data on the photoreceptor drum by scanning thereon, a developing unit for developing a latent electrostatic image formed on the photoreceptor drum surface, and the like. Color images of different colors formed by those image forming units are sequentially transferred on a recording medium, which is transported by a transport unit as a belt, thereby superimposing the color images into a color image.
This color image forming apparatus can effectively form a color image since the color image is formed by sequentially transferring the color images of different colors formed by the image forming units on a recording medium. However, the apparatus is disadvantageous for the following reason. It is difficult for image transfer to exactly position the color images formed by the image forming units on the recording medium being transported by the transport unit. When the color images are positioned not exactly, the transferred color images are in the wrong position on the recording medium. The resultant color image has unsatisfactory color quality because of its minute out-of-registration of colors.
The wrong position of the transferred color images is caused by rotation speed error of the photoreceptor drums, speed error of the transport unit, error of the image writing timings of the image forming units, and further minute variation among the image forming units or deformation of the units per se, that are produced when ambient conditions of external force applied to the apparatus, temperature, and the like are varied, variation of timings for various controls, and others.
With regard to the cause of the rotation speed error, the out-of-registration of colors can be reduced up to a tolerable level by properly adjusting the rotation speeds of the photoreceptor drums in a factory before delivery of the apparatuses. For the cause of the ambient conditions, also after the apparatuses are sold, the ambient conditions of forces acting on the apparatuses per se, temperature and the like vary day by day when the apparatuses are used. Thus, only the adjustment in factories is insufficient in obtaining a stable and good quality of the reproduced color image.
To solve the above problems, there are proposed techniques as disclosed in Japanese Patent Unexamined Publication Nos. Sho. 63-271275 and Hei. 1-281468. Each proposed color image forming apparatus includes a plurality of image forming units for forming a visual image corresponding to the image information of an image on an original document and a visual image of a position detecting mark as well, a moving member for successively moving a transfer area to which the visual image corresponding to the image information of the original document image and the visual image of the position detecting mark are transferred, and a mark detecting unit, located downstream in the transfer area as viewed in the moving direction of the moving member, for detecting the position detecting mark transferred onto the moving member, wherein the image forming units are controlled according to a detection signal output from the mark detecting unit so as to correct the out-of-registration of the transferred and superimposed images.
However, the conventional techniques have the following problems. As described above, in each proposed color image forming apparatus, a visual image of a position detecting mark is formed on the moving member by each image forming unit. The position detecting mark is detected by the mark detecting unit located downstream as viewed in the moving direction of the moving member. Each image forming unit is controlled according to the signal output from the mark detecting unit so as to correct the out-of-registration of the transferred and superimposed images.
Usually, a transport belt for transfer material is used for the moving member. The mark detecting unit is disposed at a location where the image transfer to the lowest part of the moving transport belt is completed. The mark detecting unit is usually a reflection type sensor which illuminates the surface of the belt and senses light reflected by the belt.
In the case of the transport belts of some specific colors, the difference between the reflectivity of the toner color forming the position detecting mark and that of the belt surface is small. Under this condition, the mark is frequently mistakenly detected.
Japanese Patent Unexamined Publication No. Hei. 1-270073 discloses a technique for solving this problem of mistakenly detecting the mark. In the technique, the mark detecting unit operates in response to light of a specific wavelength for detecting the mark.
The spectral reflectance of four colors, yellow (Y), magenta (M), cyan (C), and black (K), that are used for color image formation, are different from one another. The sensitivity of the sensor used for the mark detecting unit varies depending on the wavelength of sensed light. Accordingly, if the sensor operates in response to light of a specific wavelength, the light sensing conditions are different according to those colors. The output signals of the sensor are not uniform. The result is the complexity of the mark position detecting process. To secure an accurate mark detection of each toner color, the specific wavelength of light must be changed every toner color. This needs a color sensor, a color filter, and the like. To handle light of different wavelengths, a broad range of wavelengths is required for the illuminating unit. The light source that can be used is limited to a specific light source, e.g., a halogen lamp. The construction of the mark detecting unit is complicated. When the halogen lamp is used, electric power is greatly consumed, and a measure for heat is required because it is located near the belt.
As the color image forming apparatus for detecting the position detecting mark by a transmission type illumination, those disclosed in Japanese Patent Unexamined Publication Nos. Sho. 63-300259 and Sho. 63-300260 have been proposed. Each of the disclosed color image forming apparatuses includes an image bearing member for bearing an image, a moving unit moving so as to transfer the image on the image bearing member at a transfer location, a registration mark forming region consisting of a transparent member provided on the moving unit, a detecting unit for detecting a registration mark formed in the registration mark forming region, and a correcting unit for correcting the position of the image on the image bearing member according to the signal output from the detecting unit.
In the proposals of the color image forming apparatuses, any specific construction of the mark detecting unit is not referred to. The registration mark forming region consisting of a transparent member must be additionally provided on the moving unit. Accordingly, the width of the moving unit is increased by the space of the registration mark forming region, so that the apparatus size is increased. The moving unit must be combined with another material forming the transparent member, thereby possibly making it difficult to stably support and transport the moving unit in a state that the moving unit is tensioned.
The present market demand constantly pressures reduction of the apparatus size. In consideration of this, the mark detecting unit must be designed so as not to affect a great influence on the apparatus size. Usually, the mark detect unit is located at the location where the image transfer to the lowest part of the moving transport belt is completed. Accordingly, it is brought into line with the plurality of image forming units. This layout of those components increases the apparatus size. There are many proposals in which the position detecting mark is detected at the roller portion supporting the belt located at the lowest part of the moving belt. The lowest roller portion, located near the heat fixing stage, is affected by heat from the fixing stage, guaranteeing an unsatisfactory accuracy of the mark detecting unit. This problem could be solved if the mark detecting unit is located apart from the fixing stage within an allowable distance. In the case of the mark detecting unit of the reflection type, its width as viewed in the transporting direction is long. Accordingly, the apparatus size is inevitably increased.